Here is my design, the baffle height is up to you, though I can make some suggestions if you want, it'll just change the height of your water. This is based on the average 15 gallon tank which according to google is 24" x 12" x 12". Now on to to:
Each letter represents the intended purpose of a chamber
A - intake, here you will have the plumbing from the display dump into the tank, you won't need a filter sock because of part B
B - sponge, of varying size, it will need to be cleaned often to prevent water from clogging but will get all the particulate matter
C - refugium, this is a 10" x 12" x ~9", I'm not sure if thats big enough to have a mantis shrimp housed comfortably, but its a big as I dare make it. I'd recommend running cheato and a mangrove or two in here. Its big enough to someday add a frag rack here too
D - media chamber, putting bags of activated carbon, purigel, or seagel or whatever you use here will clean your water. Use biomedia at the bottom.
E - just to help reduce microbubbles, add some additional sponge here to make sure nothing big gets into chamber F
F- return chamber, I made this a bit big, why? Most filter pumps are bigger than you'd expect, it's nice to have some room. I wanted to make it 1-2" larger but it would have meant downsizing the refugium. You also have room to add additional pumps for media canisters if you ever want to add them.
So what do you guys think?